Drilling fluid is circulated through the wellbore while rotary drilling or when renovating old wellbores. The drilling fluid performs the following functions:    (1) Drilling fluid transports drill cuttings from the bottom hole to the surface;    (2) Drilling fluid prevents formation fluid from spouting by balancing pressure in the borehole;    (3) Drilling fluid forms a thin but tough filter cake of mud deposited on the borehole wall to minimize formation damage;    (4) Drilling fluid cools and lubricates drilling tools; and    (5) Drilling fluid keeps drill cuttings suspended in mud even when circulation flow of the drilling fluid is paused.
Drilling fluid is classified into water-based drilling fluid and oil-based drilling fluid. A typical drilling fluid is a drilling mud which contains an adjusting agent such as dispersing agent, weighting material, polymers, salts, lubricant and so on depending on the borehole condition, in addition to bentonite mud consisting mainly of water-bentonite suspension.
Bentonite contains clay mineral called montmorillonite as a main component, and other minerals such as feldspar and quartz. The bentonite increases the viscosity of drilling fluid to transport drill cuttings to the surface of the earth effectively. Also, bentonite is deposited on the borehole wall during drilling to make a mud wall free from formation damage. Since drilling mud pressure is usually controlled to be larger than stratum pressure in the borehole, clay mineral (bentonite) contained in the drilling mud flow in the borehole is allowed to enter into pores of the stratum and thereby mud filter cake is made on the surface of the borehole wall while drilling. Thus mud filter cake can minimize formation damage occurred in the drilling operation, and also reduce friction between a drill string and borehole wall.
However, drilling mud prepared by merely adding bentonite to water has drawbacks such as low collapse resistance, weak against salt or cement, or temperature instability. For these drawbacks, the employment of the drilling mud is limited for drilling shallow layer. In order that a drilling mud can perform functions (1) through (5) described above in drilling in greater depth or underground continuous wall building construction, various adjusting agents must be further added to the drilling mud.
Typical additives include salts such as potassium ion, and polymers such as acrylate-based polymer, carboxymethyl cellulose, polysaccharide, or polyvinyl alcohol (PVA).
Potassium ion, which is a cation adsorbed electrically to montmorillonite, is known for significantly reducing swelling of montmorillonite. In addition, it is known that the above-mentioned polymers can provide a drilling mud consisting of bentonite and water with resistance against cement contamination, and can act a dispersing agent for drilling mud, thereby inhibiting increase of drilling mud viscosity.
As the polyvinyl alcohol, carboxylic acid-modified TVA as an anion-modified PVA and sulphonic acid-modified PVA having a specific structure are disclosed in, for example, JP S57-23671A (patent document 1) and in JP H08-85710A (patent document 2) respectively. In addition, US2007/0129258 (patent document 3) teaches that use of cation-modified PVA with shale hydration inhibitor can prevent hydration swelling of shale clay and adjust rheological property of the drilling fluid.
However, such anion-modified PVA and cation-modified PVA have a functional group such as sulfonic acid group or ammonium salt, which may react other additives in the drilling fluid. When the reaction occurs, the modified PVAs will not perform their essential functions in the drilling fluid.
In view of avoiding the reaction between polyvinyl alcohol and other additive, nonionic TVA is likely to be preferably employed. As an example of nonionic PVA other than unmodified PVA used for drilling fluid, oxyalkylene-modified PVA is suggested in JP2011-57769A (patent document 4).